Voyager 2 at Neptune: A Silver Anniversary

Here's a remarkable coincidence: August 25th will be remembered as the day when two NASA spacecraft passed the orbit of Neptune. The difference — and it's a big one — is that Neptune was nowhere in sight when New Horizons swept over the threshold about an hour ago, but it was very near when Voyager 2 made its historic crossing exactly 25 years ago.

(Technically, the coincidence isn't exact. Flight engineers calculate that New Horizons crossed Neptune's orbit at 10:04 p.m. Eastern Daylight Time, whereas Voyager 2's flyby came at 3:57 Universal Time, which is 11:57 p.m. on the previous day. That's almost 24 hours' difference, but it's close enough, I suppose.)

Billions of miles from home, the Voyager spacecraft (one illustrated here) are still collecting data.NASA / JPL

And "close" is what flight engineers had in mind when they planned Voyager 2's Neptune encounter was its fourth and final planetary flyby. The spacecraft swept by the blue-and-white clouds capping Neptune's north pole from just 3,050 miles (4,900 km) away — a nearness made necessary so that its trajectory could be angled downward sharply to skim past the big moon Triton just 5¼ hours later.

I recall all these details vividly because on that day I was sitting anxiously at the Jet Propulsion Laboratory awaiting the outcome of events taking place 3 billion miles away. Despite having an oversized radio dish 12 feet across, the spacecraft could only relay 21,600 bits of data to Earth per second in a faint electronic whisper of just 10–16 watt.

Neptune's Surprises

That trickle of data carried a torrent of amazing discoveries. For Sky & Telescope's February 1990 issue, the first with detailed results, devoted 20 pages to the flyby's images and scientific findings.

Voyager 2 imaged Neptune when it flew by in August 1989. Methane gas, which preferentially absorbs red light, causes the blue hue. The dark spot in the middle is a cyclonic storm as wide as Earth.NASA / JPL

For example, mission scientists found themselves peering into the Great Dark Spot, a giant cyclonic storm in Neptune's atmosphere as large as Earth. The giant planet, hued blue due to methane in its upper atmosphere, is bathed in sunlight just 1⁄900 the intensity that we enjoy here. Yet Neptune sported many bright and dark cloud features — a far more interesting assortment than the blandness found 3½ years earlier at Uranus.

The craft's onboard magnetometer and plasma-wave instrument revealed that the Neptune's powerful magnetic field was both crazily tipped with respect the planet's spin axis (by 47°) and inexplicably offset from its core by nearly half of the planet's radius.

Astronomers already knew that Neptune has rings before Voyager 2's arrival, but it's a weird system peppered with a few dense sausages of particles, called "arcs", strung along a series of tenuous nested bands. The craft's discovery of six moonlets included one that likely was discovered from Earth during a long-forgotten occultation witnessed from Earth in 1981.

Voyager 2 made the most of its time near the giant moon Triton, which turned out to be an ice-capped wonderland of terrain types. Here and there, cameras captured gas-powered geysers squirting dark plumes several miles into the sky — and much of the surface had a nubby texture reminiscent of a cantaloupe's skin.

This week Paul Schenk (Lunar & Planetary Institute) unveiled an improved view of Triton created by reprocessing Voyager 2's quarter-century-old imagery. Check out the video he made, which recreates the historic 1989 flyby:

Thanks to Voyager 2, we know now that Triton is a very close match to Pluto in size, density, and overall composition. Planetary scientists who study Pluto are eager to see just how much of a resemblance there'll be when New Horizons flies past this little dwarf planet world in less than a year's time.

Luck of the Draw

Voyager 2 was only able to reach Uranus and Neptune thanks to a fortuitous outer-planet alignment that recurs only once in 176 years. Such a "grand tour" was made possible by using close flybys of Jupiter, then Saturn, to provide a healthy boost of speed as the craft slingshot farther outward.

The plan called for Voyager 1 to deviate at Saturn so it could flyby past the planet's giant moon Titan before eventually leaving the heliosphere for interstellar space. Its twin, Voyager 2, got the nod to try to fly past all four of the giant outer planets.

But it's a little-known secret that the Uranus and Neptune encounters might never have happened, save for a fluke in their launch order. Voyager 2 actually left Earth first, on August 20, 1977, thrust into space by a powerful Titan-Centaur booster. Keeping the two outer planets within reach required maximum performance from the launch rocket — and it delivered.

Voyager 1 left Earth 16 days later, by which time the planetary-alignment window had closed. But Voyager 1's Titan booster shut down prematurely, and its Centaur upper stage was only barely able to compensate for the lost thrust — reaching escape velocity just 3.4 seconds before running out of fuel. It still reached Jupiter and Saturn as planned. However, had the underachieving Titan been used for Voyager 2 instead, the Uranus and Neptune encounters would have been lost. By pure chance, Voyager 2 got the better rocket.

The skills and ingenuity of our spacecraft designers, and their controllers, never ceases to amaze me! Just think about this fact alone: Because the amount of sunlight at Neptune is about 1/900th of what we have here, the exposures for the remarkable photos that Voyager during its flyby took several hours. The spacecraft was zooming along at about 35,000 +/- mph so the camera had to turn to stay on target during those lengthy exposures to ensure photographic clarity. That’s just remarkable! Sometimes I take blurry photos even though both I, and my subject, are stationary!

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